Tuning system with provisions for calculating the local oscillator frequency from an aft characteristic

ABSTRACT

A television tuning system includes control apparatus for changing the frequency of a local oscillator signal during a search and a comparator arrangement for determining when an AFT signal traverses first and second threshold values corresponding to first and second frequencies in respective &#34;hump&#34; regions of the AFT signal. The frequency of the local oscillator is set to the average of the first and second frequencies in order to set the frequency of the IF picture carrier closer to the nominal value than the first and second frequencies.

This application is a continuation-in-part of application Ser. No.149,783 filed Jan. 29, 1988 in the name of Juri (NMN) Tults, nowabandoned, which itself is a division application of Ser. No. 047,848filed May 8, 1987 in the name of Juri (NMN) Tults, now U.S. Pat. No.4,823,387.

FIELD OF THE INVENTION

The present invention concerns a tuning system including automatic finetuning (AFT) provisions.

BACKGROUND OF THE INVENTION

In tuning systems employed in radio and television receivers, a receivedRF signal with an information bearing carrier is heterodyned with alocal oscillator signal to produce an IF signal with an informationbearing carrier corresponding to that of the IF signal. An automaticfine tuning (AFT) signal, having a polarity and magnitude with respectto a reference level representing the polarity and the magnitude of adeviation of the frequency of the IF signal with respect to a nominalvalue corresponding to correct tuning, is usually used to control thefrequency of the local oscillator signal to achieve fine tuning so as tobring the frequency of the IF carrier as close as possible to thenominal value. Typically, the AFT signal has an S-shaped characteristicwith positive-going and negative-going humps on opposite sides of atransition region containing the nominal frequency of the IF carrier.Tuning is considered at least approximately correct if the frequency ofthe IF carrier is within the transition region.

Many tuning systems are known which change the frequency of a localoscillator signal in a step-wise search until an amplitude comparatorarrangement determines that one or more threshold levels of an AFTsignal are achieved or traversed. For example, it is known to terminatea search when a threshold level corresponding to one of the two "humps"of the AFT characteristic is traversed in a direction going toward thetransition region between the humps.

SUMMARY OF THE INVENTION

It is recognized here that it is desirable and possible in the type oftuning system referred to above which includes a comparator arrangementfor detecting first and second frequencies at which an AFT signaltraverses first and second threshold levels, e.g., corresponding to the"humps" of the AFT signal, to control the frequency of the localoscillator signal so that the frequency of the IF carrier is set closerto the nominal frequency than the first and second frequencies. Inaccordance with the invention, the latter is achieved by calculating thefrequency of the local oscillator signal corresponding to a correcttuning condition in accordance with a predetermined computationutilizing both of the first and second frequencies. In the preferredembodiment the computation produces the average of the first and secondfrequencies.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a block diagram of a television receiver employing a tuningsystem constructed in accordance with the present invention;

FIG. 1a shows an AFT signal waveform useful in understanding an aspectof the tuning system shown in FIG. 1;

FIGS. 2a and 2b show waveforms indicating a tuning algorithm for thetuning system shown in FIG. 1;

FIGS. 3a and 3b show flow charts of portions of the program for themicroprocessor of the tuning system shown in FIG. 1 for establishing thetuning algorithm shown in FIGS. 2a and 2b including a feature of thepresent invention; and

FIG. 4 shows a flow chart of a modification of the program shown inFIGS. 3a and 3b; and

FIG. 5 shows a flow chart of a modification of the program shown inFIGS. 3a and 3b including another aspect of the present invention.

DETAILED DESCRIPTION OF THE DRAWING

The television receiver shown in FIG. 1 includes an RF input 1 which maybe connected either to a broadcast receiving antenna for receiving"off-the-air" RF signals associated with respective broadcast or "air"channels, to a cable distribution network for receiving RF signalsassociated with respective "cable" channels, or to a televisionaccessory such as a video tape machine, video disc player, video camera,home computer or video game. RF input 1 is connected to a tuner 3.Sometimes, a television accessory such as a video tape machine or avideo disc player is intended to be connected between the broadcastreceiving antenna or cable distribution network and RF signal input 1and includes a RF switching network for selectively providing either theRF signals from the connected one of the broadcast receiving antenna orthe cable distribution network, or the RF signal from the televisionaccessory to tuner 3.

Tuner 3 is capable of tuning either air channels or cable channels. Suchtuners are well known in the art and are sometimes referred to as being"cable-ready" or "cable-compatible". Although not shown, as is wellknown, tuner 3 includes an RF stage and local oscillator responsive toband selection signals and to a tuning voltage for converting(heterodyning) the RF signal associated with a selected channel to acorresponding IF signal. The band selection signals determine the tuningconfiguration of the RF stage and the local oscillator according to thetuning band of the selected channel. The magnitude of the tuning voltagedetermines the RF signal selected by the RF stage and the frequency ofthe local oscillator.

The IF signal is processed in conventional fashion in an IF section 5and coupled to a signal processing section 7. Signal processing section7 demodulates the modulated picture and sound carriers of the IF signalto produce baseband video and audio signals at respective outputs.

An automatic fine tuning (AFT) signal representing the deviation, ifany, of the frequency of the picture carrier of the IF signal from anominal frequency value, e.g., 45.75 MHz in the United States, isgenerated by an AFT detector 9. The typical S-shaped waveform of the AFTsignal is shown in FIG. 1a. The polarity of the AFT signal relative toan amplitude level corresponding to the nominal frequency represents thesense of the frequency deviation with respect to the nominal frequencyand the amplitude of the AFT signal represents the magnitude of thefrequency deviation. By way of example, in the present embodimentnegative-going excursions below the amplitude level corresponding to thenominal frequency correspond to negative frequency deviations andpositive-going excursions correspond to positive frequency deviations.The AFT signal is utilized in the tuning process as will be describedbelow.

A composite synchronization ("sync") signal is derived from the videosignal by a sync detector 11. In addition to its ordinary use of picturesynchronization, the composite synchronization signal is also utilizedin the tuning process as will also be explained below.

The tuning voltage for tuner 3 is generated by a tuning voltagegenerator 13 in response to a digital signal related to the selectedchannels. Tuning voltage generator 13 may be of the voltage synthesistype including a digital-to-analog converter or of the frequencysynthesis type including a frequency or phase locked loop. In thepreferred embodiment, tuning voltage generator 13 is of the frequencysynthesis type because of the inherent accuracy and stability of thetype of system. A suitable frequency synthesis type of tuning voltagegenerator including a phase locked loop (PLL) is described in U.S. Pat.No. 4,405,947 issued in the name of J. Tults and M. P. French. Asuitable frequency synthesis type of tuning voltage generator includinga frequency locked loop (FLL) is described in U.S. Pat. No. 4,485,404issued in the name of J. Tults on Nov. 27, 1984. By way of example, itis assumed that a PLL tuning voltage generator is employed.

Briefly, a PLL tuning voltage generator includes a cascade of a fixedfrequency divider (usually referred to as a "prescaler") for dividingthe frequency of the local oscillator signal by a factor K and aprogrammable frequency divider for dividing the frequency of the outputsignal of the prescaler by a programmable factor N. A fixed frequencydivider divides the frequency (f_(XTAL)) of the output signal of acrystal oscillator by a factor R to derive a reference frequency signal.A phase comparator compares the output signal of the programmabledivider to the reference frequency signal to generate an "error" signalrepresenting the phase and frequency deviations between the outputsignal of the programmable divider and the reference frequency signal.The error signal is filtered to produce the tuning voltage. The tuningvoltage controls the frequency (f_(LO)) of the local oscillator until:##EQU1## Thus, the frequency of the local oscillator signal can becontrolled by controlling programmable factor N. If K, R and f_(XTAL)are selected so that K/R f_(XTAL) equals 1 MHz, N is equal in MHz, tothe frequency of the local oscillator signal. Division factor N iscontrolled in response to the selected channels and to locate and tunenon-standard frequency RF signals as will be explained below.

A microprocessor 15 generates a digital representation of theprogrammable factor N for controlling the frequency of the localoscillator signal and the band selection signals for tuner 3.Microprocessor 15 operates under the control of a computer programstored in a read-only-memory (ROM) 17. The portion of the programgermane to the present invention is shown in flow chart form in FIG. 3.Microprocessor 15 responds to user command signals generated by a usercontrol keyboard 19. Although keyboard 19 is shown directly connected tomicroprocessor 15 for simplicity, it may comprise the keyboard of aremote control unit.

Keyboard 19 includes keys for controlling various functions of thetelevision receiver such as turning the receiver "on" and "off",controlling the volume level, and selecting channels to be tuned. Onlythe keys germane to channel selection are shown.

Digit keys (0-9) are provided for directly selecting a channel byentering the tens and units digits of the respective two digit channelnumber.

"Channel up" (CUP) and "channel down" (CDN) keys are provided forinitiating a "channel scanning" mode of channel selection in whichchannels are successively tuned in increasing or decreasing frequencyorder until a channel in a list of active channels is located. Thosechannels not in the list will be automatically skipped over during thechannel scanning mode. The list of active channels is stored in anon-volatile random access memory (RAM) 21 associated withmicroprocessor 15. RAM 21 includes a plurality of one-bit memorylocations for respective channels. A logic "1" is stored in the memorylocations for each active channel and a logic "0" is stored in thememory locations for each inactive channel. The memory locations areaddressed in accordance with the channel number of the selected channel.

Keyboard 19 also includes an "air/cable" (A/C) key for selecting whichof air or cable channels are to be tuned. A single-bit indication (e.g.,a logic "1" for air channels and a logic "0" for cable channels) ofwhether air or cable channels are to be tuned is stored in RAM 21.

Keyboard 19 additionally includes an "auto-program" (A-P) key forinitiating an "auto-programming" mode for automatically "programming"the active channel list of RAM 21. During the auto-programming mode, allthe channels are successively selected for tuning and at each channel itis determined, as described below, whether or not a valid RF televisionsignal is present. A logic "1" is entered into the respective one-bitmemory location of RAM 21 if a valid RF television signal is present anda logic "0" is entered if a valid RF television signal is not present.

The user may not want to receive all the active channels located duringthe auto-programming mode. On the other hand, the user may want tuningto stop at certain channels, such as channels used for a video cassetterecorder, video game or home computer, which may not be continuouslyactive or which may not be located for other reasons, as will bediscussed below, during the auto-programming mode. For these reasons,keyboard 19 also includes "erase" and "add" keys for manually deletingand adding channels from the list stored in RAM 21. The digit keys maybe used in conjunction with the "erase" and "add" keys to deletechannels and to add channels to the list stored in RAM 21.

As earlier noted, RF input 1 may be connected to an air, a cable, or atelevision accessory RF signal source. The RF signals for air channelsoccupy low VHF, high VHF and UHF tuning bands and have carriers withstandard frequencies assigned by the FCC. The RF signals for cablechannels may also occupy the low VHF, high VHF and UHF bands and inaddition may occupy mid, super, hyper and ultra bands interspersed withthe low VHF, high VHF and UHF bands. The same channel numbers identifydifferent air and cable channels. The RF signal produced by a televisionaccessory is usually selectively available at one of two VHF channels,e.g., channels 3 and 4.

In view of the foregoing, the band selection signals for selecting thetuning configuration of the RF stage and local oscillator of tuner 3 arecontrolled in response to the air/cable selection indication stored inRAM 21 as well as by the channel number of the selected channel. Inaddition, microprocessor 15 translates channel number of a channelselected for tuning to the appropriate division factor N and translatesthe channel numbers successively generated during the normal ("channelup" and "channel down") and auto-programming channel scanning and to theappropriate memory address for RAM 21 depending the air/cable selectionindication.

The values of the division factor N for air channels with standardfrequency RF signals are known in advance for every receiving location.Therefore the precise value of N for each air channel can be stored aspart of the control program for microprocessor 15.

However, the values of division factor N for cable channels andtelevision accessories with non-standard frequency RF signals which maybe offset from respective standard frequencies are not known in advancefor every receiving location. Therefore, the particular values of N forcable channels and television accessories cannot be stored in advance.Rather, when a cable distribution network or television accessory isconnected to RF signal input 1, a search for the correct value of N isconducted for each channel to be tuned. During this search, the value ofN is changed in steps in a range around the value of N for a respectivestandard frequency and, at each value of N, it is determined whether ornot a valid television RF signal is present.

The searching provisions are desirable both in the normal tuning mode(in which channels are selected either directly with the tens and unitsdigit keys or indirectly with the "channel up" and "channel down" keys)or during the auto-programming mode. It is recognized here that whilethe search should be as complete as possible to be able to tune almostany non-standard RF signal, the use of such complete searches,especially in the auto-programming mode tend to require an excessivelylong time. Some prior tuning systems have auto-programming provisionslimited to locating only channels with standard frequency RF signals.While such limited auto-programming provisions are not time consuming,they will locate only active channels having carriers with non-standardfrequencies at or very near respective standard frequencies. This isundesirable in view of the growing access to cable distribution networksproviding a very large number of active channels.

To shorten the time to tune non-standard frequency RF signals during thenormal tuning mode or to locate them during the auto-programming mode,the present tuning system takes advantage of the recognition that whilecable distribution networks provide RF signals having carriers withnon-standard frequencies, the most commonly encountered non-standardfrequencies can be categorized into a few groups of predictablefrequencies. Specifically (as shown in FIGS. 2a and 2b), the tuningalgorithm for searching for non-standard frequency RF signals ispartitioned so that the presence of an RF signal is tested for initiallyat each of a first relatively small group of values of N correspondingto respective local oscillator search frequencies corresponding topredictable non-standard frequencies commonly encountered in cabledistribution networks and thereafter at each of a second, relativelylarge, group of values of N corresponding to local oscillator searchfrequencies. The search algorithm is relatively quick because the mostcommonly encountered, and therefore most likely, non-standard RF signalsare looked for first. During normal tuning modes, in which only a singlechannel is selected for tuning, both groups local oscillator searchfrequencies are utilized. However, during the auto-programming mode, inwhich all the channels are successively selected for tuning, only thefirst group is utilized.

The present tuning algorithm has been found to significantly reduce thetime required for the auto-programming mode. While some active channelssuch as these corresponding to television accessories may not be locatedduring the auto-programming mode, this is not a serious deficiency forseveral reasons. First, it is not likely that a television accessorywould be activated during an auto-programming operation and thereforethe associated channel would not be identified as being active by eventhe most complete search algorithm. Second, the channel associated witha television accessory will usually be known in advance and can bemanually added to the list of active channels.

The present invention will now be more specifically described withreference to the cable distribution networks employed in the UnitedStates. The major cable distribution networks employed in the UnitedStates utilize one of the following three frequency allocation systems:

1. Standard Cable System--The frequencies of the picture carriers forchannels 2 to 6 and 7 to 13 are at the FCC assigned broadcast (standard)frequencies. Additional channels are provided with carriers at 6 MHzintervals between 91.25 MHz and 169.25 MHz and between 217.25 MHz and643.25 MHz.

2. HRC (Harmonical Related Carriers) System--The frequencies of thepicture carriers of all the channels, except channels 5 and 6, havefrequency offsets of 1.25 MHz lower than respective frequencies of theStandard Cable System. The frequencies of the carriers for channels 5and 6 are 0.75 MHz higher than respective frequencies of the StandardCable System.

3. IRC (Interval Related Carriers)--The frequencies of the carriers ofall the channels, except channels 5 and 6, are not offset fromrespective frequencies of the Standard Cable System. The frequencies ofthe carriers for channels 5 and 6 are 2.0 MHz higher than respectivefrequencies of the Standard Cable System.

Accordingly, for use in the United States, for all channels selected fortuning, except channels 5 and 6, the first group of search frequenciescorrespond to:

1. The local oscillator frequency for the standard frequency (NOM) RFpicture carrier; and

2. NOM--1.25 MHz

For channels 5 and 6, the first group of search frequencies correspondto:

3. NOM;

4. NOM+0.75 MHz; and

5. NOM+2.0 MHz

Actually, as shown in FIGS. 2a and 2b, the search frequencies in thefirst group are pairs of frequency, each pair corresponding to one ofthe frequencies identified above. The reasons for the pairs stems fromthe use of the AFT signal to indicate the presence of valid RFtelevision signals as will be explained below.

In the present tuning system, the presence of a valid RF televisionsignal is determined by examining the conditions of the AFT and/or thecomposite sync signal. AFT comparators 23a and 23b and a sync comparator25 coupled to microprocessor 15 are provided for this purpose.

The normal tuning mode for tuning a channel will be explained first. Itis assumed that a cable network is connected to RF input 1 and that theair/cable key has been operated to place the tuning system in conditionfor tuning cable channels. During the following discussion, reference toFIGS. 2a and 2b and FIG. 3 should be made.

As shown in FIG. 1a, the AFT signal has a positive-going hump above alevel V_(H) with a peak at approximately 125 kHz above nominal frequencyof the picture carrier and a negative-going hump below a level V_(L)with a peak at approximately 125 kHz below nominal frequency f. Thedetection of the positive and negative-going humps by AFT comparators23a and 23b, respectively, indicates the presence of a RF carrier forthe selected channel. The order in which the humps are detected relativeto the direction of frequency change is important in properlyidentifying the presence of a valid RF television signal. For thedecreasing frequency direction of the local oscillator signal (andtherefore of the IF signal) in the present embodiment, thepositive-going hump (indicating a positive frequency deviation) isencountered before the negative-going hump (indicating a negativefrequency deviation). The reverse is true for the increasing frequencydirection. The V_(H) and V_(L) threshold voltages applied to comparators23a and 23b correspond to the V_(H) and V_(L) levels of the AFT signaldefining the positive and negative-going humps as shown in FIG. 1a.Accordingly, during the search of the first group of local oscillatorsearch frequencies, the values of division factor N are set to produce±125 kHz pairs of frequencies with respect to the five local oscillatorsearch frequencies identified above.

If both the positive-going and negative-going AFT humps are not detectedby AFT comparators 23a and 23b at a particular value of N, N is changedto the next search value in the first group of search values. If bothAFT humps have not been detected for any search value in the firstgroup, it indicates that a valid RF television signal has not been foundfor the search values of the first group. In that case a so-called "syncedge" search utilizing the search values in the second group isinitiated. The "sync edge" search will be explained below.

If both the AFT humps have been detected for a search value in the firstgroup, the search is terminated and the composite synchronization signalis examined with sync comparator 25. The composite synchronizationsignal is examined because it is possible that the carrier detected byAFT comparators 23a and 23b may be a sound carrier rather than a picturecarrier. A suitable sync validity detector which operates by measuringthe frequency and pulse width of the pulses of the composite sync signalis described in the aforementioned Tults patent.

If the composite synchronization signal has the correct characteristics,the picture carrier of a RF television signal has been located and thesearch is terminated. However, the frequency of the local oscillatorsignal is adjusted for optimum tuning. That is, the frequency of thelocal oscillator signal is not left at the frequency at which the last(i.e., negative-going) hump was located since that last local oscillatorfrequency corresponds to an IF picture carrier frequency which isremoved from the nominal frequency. Rather the final frequency of thedetected IF picture carrier is set between the two humps, and thereforemuch closer to the nominal IF picture carrier frequency, by setting thelocal oscillator frequency to the average of the frequencies at whichthe positive-going and negative-going humps were located. Thereafter,the frequency of the IF picture carrier is maintained between the twohumps by comparing the amplitude of the AFT signal against thresholdlevels V_(L) and V_(H) and, if one of the two threshold levels istraversed, adjusting the frequency of the local oscillator signal insmall steps, e.g., 31.25 kHz, in the opposite direction until thetraversed one of the two threshold levels is again traversed.

If the composite synchronization signal does not have the correctconditions, it also indicates that the picture carrier of a RFtelevision signal was not located for the first group of searchfrequencies. As in the case when the negative and positive-going humpsare not detected, the "sync edge" search is initiated.

During the "sync edge" search, sync comparator 25 is used to examine thecomposite synchronization signal at each of the search frequencies ofthe second group. As shown in FIGS. 2a and 2b, in the presentembodiment, this search occurs at 0.5 MHz steps and starts at a localoscillator frequency 4.0 MHz higher than the nominal frequency and endsat local oscillator frequency 3.0 MHz lower than the nominal frequency.The presence of a picture carrier of an RF television signal isindicated when a valid composite sync signal was not detected(indicated, e.g., by the generation of a logic "0" by composite syncdetector 25) for the previous step and a valid composite sync signal isdetected (a logic "1") for the present step. At that point, the localoscillator frequency corresponding to optimum turning is no more than0.5 MHz from the present step. The term "sync edge" corresponds to thetransition from the invalid sync condition to the valid sync conditionbetween steps. If the direction of search were reversed, the test forthe location of a picture carrier would be a transition from a validcomposite sync condition to an invalid sync condition.

The "sync edge" search is utilized because it more precisely locates thepicture carrier than by merely determining when the compositesynchronization signal is valid. This is so because the synchronizationsignal is valid for a very wide range (greater than 0.5 MHz) of localoscillator frequencies surrounding the local oscillator frequencycorresponding to optimum tuning. Thus, utilizing sync comparator 25alone (without regard to the transition between steps) could produce alocal oscillator frequency considerably removed from the optimum value.

After the presence of the picture carrier of a valid RF signal has beenlocated, the same operation used to maintain the frequency of the IFpicture carrier between the AFT humps as previously discussed isutilized to optimize the tuning.

If an RF carrier is not located during the "sync edge" search, thenominal local oscillator frequency corresponding to the standardfrequency RF signal for the selected channel is caused to be generated.

In the present tuning system, the search provisions are not defeated forair channels in order to be able to tune a non-standard frequency RFsignal provided by a television accessory which is connected, in themanner described above, between a broadcast receiving antenna (whichprovides only standard frequency carriers) and the receiver. However,since the picture carrier will not likely be found for one of thepredictable frequencies of the first group associated with cablechannels, these frequencies are not examined for tuning air channels.

It is noted that if the AFT humps have been detected for the nominallocal oscillator frequency for the selected channel and air channelshave been selected for tuning, the validity of the composite sync is notexamined if the AFT humps have been detected for the nominal localoscillator frequency. This is because, in this situation, it is veryunlikely that the carrier will be a sound carrier.

The major difference between the auto-programming mode and the normaltuning mode is that only the first group of search frequencies areexamined and the only test conducted is the one for the positive andnegative-going AFT humps (i.e., the test for the validity of thecomposite synchronization signal is not conducted). If the two AFT humpshave been detected for any search value for a channel selected to betuned during the auto-programming mode, the channel is added to the listof active channels, otherwise, is deleted.

Another difference between the auto-programming mode and the normaltuning mode is that the composite synchronization signal is not examinedin the auto-programming mode of the present embodiment to the goal ofkeeping the required time to a minimum. However, the tuning algorithmcan be simply modified by changing the program for microprocessor 15 toadd an examination of the composite synchronization signal to reduce thelikelihood of erroneously adding a channel to the list of activechannels due to the detection of positive and negative humps, e.g., inresponse to a sound carrier. However, this is not believed necessary forthe predictable search frequencies.

The AFT signal is utilized to indicate the presence of a valid RFtelevision signal during the search of the first group of searchfrequencies because the search in the first group is limited to a fewpredictable frequencies for which the presence of a valid RF televisionsignal is likely to occur. The AFT signal indicates the location of avalid television signal in a range of frequencies around the nominal IFpicture carrier frequency (e.g., +500 kHz and to -1 MHz) smaller thanthe range of frequencies (e.g., +500 kHz to -3.5 MHz) for which thecomposite synchronization signal indicates the presence of a valid RFtelevision signal. Thus, if the composite synchronization signal wereutilized without the "sync edge" search, the location of a valid RFtelevision signal could be indicated at a particular search frequencywhich is further from the optimum local oscillator frequency than thatat which the AFT signal would indicate the location of a valid RFtelevision signal. As a result more than fine tuning might be requiredto locate the optimum local oscillator frequency, as indeed is the caseif a valid RF television signal is located during the sync edge searchas previously discussed.

In the present embodiment, the "sync edge" search utilizing thecomposite synchronization signal is utilized for the second group ofsearch frequencies. The "sync edge" search has been found to veryreliably indicate presence of the picture carrier of a RF televisionsignal. However, it is believed that the search of the second group offrequencies may also be conducted by looking for the AFT humps. In thiscase the validity of the composite synchronization signal should betested if the humps are detected to avoid the possibility of identifyinga sound carrier as previously explained.

As is known, many of the channels of a cable distribution system provideRF signals with "scrambled" video components in which the horizontalsynchronization component is suppressed, inverted or otherwise modifiedrequiring a descrambler. Usually, the descrambler is located in aseparate cable converter with its own tuner. Recently, it has beenproposed that the tuner of the television receiver be utilized therebyeliminating the need for a tuner in the cable converter and alsoenabling the use of the remote control system of the television receiverto select channels. In this case, the receiver would be provided withinput and output terminals for coupling the detected video signal to andfrom an external descrambler.

Since the search of the first group of frequencies involves thedetection of the AFT humps but not the evaluation of the compositesynchronization signal in the auto-programming mode, active "scrambled"cable channels having the predictable frequencies within the first groupwill be located in the auto-programming mode. However, active"scrambled" cable channels with other non-standard frequencies will notbe located in the auto-programming mode since the evaluation of thecomposite synchronization signal is involved. In addition, active"scrambled" cable channels will not be tuned in the normal tuning modedue to the evaluation of the composite synchronization signal.

The tuning algorithm can be modified to eliminate the test for a validcomposite synchronization signal after the detection of the AFT humps inthe normal tuning mode so that "scrambled" cable channels withpredictable frequencies in the first group can be tuned. However, thisdoes not solve the problem of tuning "scrambled" cable channels withother non-standard frequencies.

FIG. 4 shows a modification for the program shown in FIG. 3 to overcomethe above problems resulting from "scrambled" cable channels. Themodification is intended to replace the "sync edge" search portion ofthe program shown in FIG. 3 which utilizes only the compositesynchronization signal. Basically, in addition to the "sync edge" test atest for a transition or cross-over from a positive-going AFT hump to anegative-going AFT hump is conducted for successive steps. Morespecifically, tests for positive and negative-going AFT humps areconducted at each 0.5 MHz step. The location of a picture carrier may beindicated by the detection of a positive-going hump at one step followedby the detection of a negative-going hump at the next step. (If thedirection of search were reversed, then a negative-going hump followedby a positive-going hump could indicate the location of a picturecarrier). The frequency at which the transition or the AFT cross-overfrom the detection of the positive-going hump to the negative-going hump(actually the frequency at which the negative-hump) is located isstored. However, the AFT cross-over frequency is not tuned until acomplete search of the second group frequencies has been completed andit is determined that no "sync edge" transition from an invalid synccondition to a valid sync condition has occurred. The "sync edge"indication of a picture carrier is therefore given priority over the AFTcross-over indication of picture carrier. This is so because normalpicture carriers are more likely to occur than scrambled picturecarriers.

To ensure that positive-going hump and following negative-going humpcorrespond to the same carrier, a test is conducted to determine if thedetected humps are separated by no more than 1 MHz (i.e., two steps).The variable PSI is used for this purpose. Note that a PSI value of 3 inthe program corresponds to 1 MHz.

To reject the detection of the sound carrier of the lower adjacentchannel, a test is conducted to determine if a second cross-over from apositive-going to a negative-going hump is detected at a frequency about1.5 MHz after the frequency of a first cross-over. The variable CSI isused for this purpose. Note that a CSI value of 5 in program correspondsto at least 2 MHz.

If a cross-over from a positive-going to negative-going hump is detectedand an invalid sync to valid sync transition has not been located afterthe complete search, the local oscillator frequency is set to frequencyat which the negative-going hump was detected. Thereafter, as shown inFIG. 3, the local oscillator frequency is modified by 31.25 kHz stepsuntil the local oscillator frequency is between the two humps.

It may be possible to eliminate the test concerning the separationbetween humps and/or the test for rejecting the sound carrier of thelower adjacent channel. In practice it has been found that the first ofthese tests may be readily eliminated.

The flow chart of a modification of the fine tuning operation shown inFIG. 3b is shown in FIG. 5.

It provides for finding the frequencies of the V_(H) and V_(L) AFTvoltage levels corresponding to the positive and negative-going AFThumps and calculating the final local oscillator frequency from thefrequencies of the V_(H) and V_(L) AFT voltage levels. This allows thefinal frequency of IF picture carrier to be set much closer to thenominal IF picture carrier after the sync edge search routine thanpossible using the operation shown in FIG. 3b (in which the finalfrequency is set to correspond to one of the V_(H) and V_(L) AFT voltagelevels). This is particularly important considering that the slope ofthe control range between the AFT humps can vary considerably.

In FIG. 5, the portions which are the same as corresponding portions inFIG. 3b are repeated for reference. The modified portion starts at pointB. First, the operation ensures that the AFT signal is above the V_(H)reference level (i.e., the picture carrier frequency is in the region ofthe positive AFT hump). Thereafter, the frequency of the localoscillator signal is reduced by small steps (e.g., 31.25 KHz) until thefrequency (f_(LOH)) for V_(H) and the frequency (f_(LOH)) for V_(L) areboth located. The final frequency is calculated as the average off_(LOH) and f_(LOL) and caused to be tuned (synthesized). If eitherf_(LOH) or f_(LOL) cannot be located, the frequency of the localoscillator will be caused to be offset by more than 3 MHz from thenominal value. In that case the nominal local oscillator frequency iscaused to be tuned.

While the present invention was described with respect to a preferredembodiment, several possible modifications were noted. These and othermodifications, such as utilizing the horizontal synchronizationcomponent of the composite synchronization signal are contemplated to bewithin the scope of the invention defined by the following claims.

What is claimed is:
 1. Tuning apparatus comprising:RF means forproviding an RF signal having an information bearing carrier; localoscillator means for generating a local oscillator (LO) signal having afrequency controlled in response to a control signal; mixer means forcombining said RF signal and said LO signal to produce an IF signalhaving an information bearing carrier corresponding to said informationbearing carrier of said IF signal; automatic fine tuning meansresponsive to said IF signal for producing an AFT signal having anamplitude versus frequency response with a polarity and levelrepresenting the sense and magnitude of a deviation of the frequency ofsaid IF carrier from a nominal value; amplitude comparator means forgenerating first and second signals when said AFT signal traverses firstand second threshold levels corresponding to respective first and secondfrequency deviations of said IF carrier from said nominal value; andcontrol means for generating said control signal to control thefrequency of said local oscillator signal, said control means causingthe frequency of said LO signal to change and thereby cause said AFTsignal to sequentially traverse said first and second threshold level,said control means being responsive to said first and second signals forlocating said first and second frequency deviations and for calculatinga frequency of said LO signal corresponding to a correct tuningcondition in accordance with a predetermined computation utilizing bothof said first and second frequency deviations.
 2. The tuning apparatusrecited in claim 1 wherein:said first and second threshold levels ofsaid AFT signal corresponds to first and second opposite senses of thedeviation of the frequency of said IF signal.
 3. The tuning apparatusrecited in claim 2 wherein:said predetermined computation comprises anaverage of said first and second frequencies.
 4. The tuning apparatusrecited in claim 3 wherein:said first sense is the positive sense andsaid second sense is the negative sense.
 5. The tuning apparatus recitedin claim 1 wherein:said control means comprises a frequency synthesizer.6. The tuning apparatus recited in claim 5 wherein:said control meanscomprises a phase locked loop.
 7. The tuning apparatus recited in claim2 wherein:said control means causes the frequency of said LO signal tochange in a number of steps greater than one to locate said secondfrequency deviation after said first frequency deviation has beenlocated.